WhatsApp uses the phone numbers saved in your phone's address book to find your friends who are already using WhatsApp. To learn more about why WhatsApp uses your phone number and address book, please read this article.
Follow these steps to chat with a friend over WhatsApp:
Note: Sometimes numbers synced from Microsoft Exchange will not work in WhatsApp.
Learn how to add contacts on: Android | iPhone | Windows Phone | Nokia S40 | Nokia S60 | BlackBerry 10
Cheers,
WhatsApp Support Team